Tumor antigens are generally proteins or glycoproteins that are present on the surface of tumor cells. In many cases, such antigens are identical to or highly similar to antigens that are present on normal, non-tumor cells in the host organism, allowing the tumor cells to escape the host's immunological surveillance mechanisms.
Traditional means of reducing tumor load in afflicted individuals have relied on chemical or radiation treatments that target particular attributes of tumor cell growth, such as hormone dependence, rate of growth, and the like. Such treatments have been shown to be effective in combatting certain types of tumors, but relatively or incompletely effective in other cases. Therefore, methods to enhance or augment the ability of an organism to immunologically eradicate some or all circulating tumor cells (tumor load) are needed.
For example, in the case of prostate tumors, although the five-year survival rates for localized prostate cancer have improved significantly, the prognosis for metastatic forms of the disease has not been improved in recent years. Prostectomy (simple or radical) and local radiation therapy are effective at early stages of the disease, but are of little or no benefit in the later, metastatic stages of the disease. Moreover, metastatic forms of prostate cancer are generally resistant to conventional anti-neoplastic chemotherapy.
The only therapy that has shown benefit so far in the disseminated form of the disease is androgen ablation, either by castration or estrogen (diethylstilbestrol) therapy. Prostate tumor cells are typically dependent on testosterone or other androgens as growth factors. However, androgen withdrawal frequently leads to outgrowth of androgen-independent, mutant tumor cells. Thus, since all currently available therapies for disseminated prostate cancer are at best palliative and do not prolong survival, improved therapies for eradicating circulating or disseminated prostate tumor cells are needed.
The present invention is concerned with an immunotherapeutic treatment method that takes advantage of the observation that is the discovery of the invention, that is, animals immunized with xenogeneic antigens can be made to mount an immune response against closely related self-antigens, such as the antigens present on tumor cells. Such a therapy has the advantages over conventional therapies that (i) it mobilizes the body's natural mechanisms for ridding itself of the diseased cells, (ii) it can be directed to disseminated forms of the disease, and (iii) it can be used to either augment or replace conventional anti-tumor therapy.